1. Field of the Invention
This invention relates to a process for preparing unsaturated carboxylic acids having from three to four carbon atoms by the catalytic vapor phase oxidation of the corresponding unsaturated aldehyde. More particularly, this invention relates to a process for preparing methacrylic acid from methacrolein, characterized mainly by the use of novel catalysts.
2. Description of the Prior Art
Many catalysts useful for the preparation of unsaturated carboxylic acids having from three to four carbon atoms by the catalytic vapor phase oxidation of the corresponding unsaturated aldehydes with molecular oxygen have already been proposed. Some of those are excellent for preparing acrylic acid from acrolein and have been used for the production of acrylic acid on a large scale. Various catalysts for preparing methacrylic acid from methacrolein have also been proposed. However, methacrylic acid has not been produced commercially from methacrolein by use of these catalysts because of the low yield. This is because methacrolein is more combustible than acrolein, i.e., it is subjected to complete oxidation to carbon monoxide and/or carbon dioxide rather than to partial oxidation to the desired product, and because methacrolein is easily polymerizable to heavy compounds. There currently exists no completely satisfactory catalyst for the preparation of methacrylic acid. U.S. Pat. Nos. 3,358,020 and 3,453,069 disclose prior art catalysts which slow excellent results for the production of acrylic acid (the conversion of acrolein is 96-100%, while the selectivity to acrylic acid is 80-90%), but not for the production of methacrylic acid (the conversion of methacrolein is 29-63%, while the selectivity to methacrylic acid is 11-41%).
A need exists, therefore, for a catalyst which minimizes or eliminates the oxidative side reactions and the polymerization reactions which occur in the conversion of acrolein and methacrolein to their respective acid derivatives.